If you believe your card number has been stolen or has the potential for misuse, then you report it to the number on the back of your current card. (Or look them up online. Just be sure you have the real site.) They will put a hold on your card so that nobody, including you, can use it until you activate the new card with the new number they will send you. The card works under the same account you previously had, however with the new number you'll need to change any recurring payments set up with the old one.